Chemical pathways of biotransformation

       Phase I and phase II metabolism

       Microsomal and non-microsomal enzymes

       First pass metabolism

Intended Learning Outcomes

At the end of this lecture, student will be able to

       Explain chemical pathways of biotransformation

       Differentiate  between phase I and phase II metabolism

       Outline microsomal and non-microsomal enzymes

       Explain the process of first pass metabolism


       Alteration of a drug within a living organism

       Metabolic transformation: Mitochondrial, microsomal, cytosolic

       Inactive to active: L- dopa – Dopamine

       Active to inactive: Chloramphenicol conjugate

       Active to active:

      More active: Diazepam – Oxazepam

      Different activity: Pethidine - Norpethidine

       Excretion unchanged – Aminoglycosides

       Metabolism : less polar, lipid soluble – more polar, water soluble

       Spontaneous change into other substances

      Mechlorethamine – active ethylniminium cations at slightly alkaline pH

      Atracurium – Hoffman reaction


      Precursor drug that itself has little or no biological activity

      Metabolized to a pharmacologically active metabolite

First pass metabolism/ Presystemic metabolism

      Before reaching systemic circulation gets metabolized

      Decreased BA, diminished therapeutic response

      Bypassed: IV, sublingual route

Chemical Pathways of Biotransformation

Phase I

       Metabolite: active/inactive

       Degradative reactions

       Small non polar/polar metabolite

       Mainly microsomal

       Few non- microsomal

Phase II

       Metabolite inactive

       Synthetic /conjugation reactions

       Polar metabolite

       Mitochondrial, microsomal, cytoplasmic

Drug Metabolizing enzymes

Microsomal enzymes

       Located on smooth ER, lungs, kidney, intestinal mucosa

       MFOs/ CYP450



       CYP2C8, 9

Non - microsomal enzymes

       Present in cytoplasm, mitochondria, plasma






Phase I reactions – oxidations

Microsomal Oxidation

       Aromatic hydroxylation

      Phenobarbitone – P-hydroxy phenobarbitone

       Aliphatic hydroxylation

      Pentobarbitone – Hydroxy pentobarbitone

       N – dealkylation

      Morphine – normorphine

      Mephobarbitone - phenobarbitone

       O -dealkylation

      Codeine - morphine

       S -dealkylation

      6-methyl thiopurine - mercaptopurine

       S – Oxidation

      Chlorpromazine - Chlorpromazine sulfoxide

       N – oxidation

      Trimethylamine – Trimethylamine N - oxide


      Amphetamine – phenylacetone derivative


      Parathion - paraoxon

Non - microsomal Oxidation

       Mitochondrial oxidation

      Adrenaline by MAO to VMA

       Cytoplasmic oxidation

      Alcohol by alcohol dehydrogenase to acetaldehyde

       Plasma oxidation

      Histamine by histaminase to IAA

Phase I reactions – Reductions

Microsomal reductions

       Nitro reduction

      Chloramphenicol to its arylamine metabolite

       Azo reduction

      Protonsil to sulfanilamide

       Keto reduction

      Cortisone to hydrocortisone

Non – microsomal reductions

Chloral hydrate to trichloro ethanol

Phase I reactions – Hydrolysis

Microsomal hydrolysis

Pethidine to pethidinic acid by hepatic esterase

Non microsomal hydrolysis

       Esterases, amidases, peptidase, protease, phosphatase

       Procaine – PABA
atropine – Tropic acid

       Hydrolysis of beta lactam ring of penicillin

Phase II Reactions

Microsomal Conjugation

       Glucuronide Conjugation:

       Parent drugs/phase I metabolites

       Containing phenolic, alcoholic, -COOH, -NH2, mercapto group

       Polar readily excreted, often inactive

       Morphine, paracetomol, aspirin, sulfonamide, bilirubin

Non microsomal Conjugations:

N –Acetyl Conjugation (In cytosol)

       Isoniazid, PAS, dapsone, sulfonamides, histamine

Sulfate conjugation (In cytosol)

       Phenolic, alcoholic, aromatic amines undergo

       Aspirin, methyldopa, paracetamol, chloramphenicol

Aminoacid conjugation (In mitochondria)

       Aspirin, benzoic acid, nicotinic acid

Methyl Conjugation (In cytosol)

       Many catecholamines, histamine

Glutathione conjugation (In cytoplasm)

       Epoxides, NO2 containing drugs, hydroxylamines

       Ethacrynic acid, sulfobromophthalein

Ribosides & riboside phosphate

       Aspirin, benzoic acid, nicotinic acid


       Phase I: Degradative reactions

       Phase II: Synthetic/ conjugative reactions

       Microsomal enzymes: Inducible

       Prodrug: Precursor drug that itself has little or no biological activity

       Glucuronide conjugation is microsomal reaction

       FPM: Before reaching systemic circulation gets metabolized

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